Provided are a battery holder, a power transfer device, an electric vehicle and an installation method for electric vehicle. The battery holder is mounted on the body of the electric vehicle to fix the battery pack, the battery holder includes a plurality of supporting devices, the plurality of supporting devices are distributed on both sides of the fixing bracket in the length direction of the fixing bracket; in the length direction of the fixing bracket, both sides of the fixing bracket are both arranged with the lock mechanism, the lock mechanism is a primary lock mechanism and a secondary lock mechanism; the secondary lock mechanism is used to provide a secondary locking function or a locking protection function for the battery pack, when the primary lock mechanism fails, it is used to prevent the battery pack from falling.